The invention is based on a hydraulic pilot control system having two control outputs (96, 97) to which a control pressure can be applied and having a hydraulic pilot controller, which has a handle (81) which can be pivoted from a neutral position in a first direction to apply a variable control pressure to the first control output (96) and in a second direction, which is preferably opposite to the first direction, to apply a variable control pressure to the second control output (97), and a pressure valve (69) which can be displaced from the neutral position by the deflection of the handle (81) and generates a control pressure at a control output (80).
Such a hydraulic pilot control system is disclosed, for example, by DE 196 30 798 A1. This pilot control system comprises a pilot controller which contains a plurality of pressure reducing valves, by which a pilot control pressure can be generated by each at a control output. The pilot controller has a control lever which, from a neutral position, can be pivoted in a first direction to adjust a first pressure reducing valve and in a second direction, opposed to the first direction, to adjust a second pressure reducing valve. In general, after the control lever has been pivoted through a specific angle in the first direction, the pilot control pressure then present on the first control output is different from the pilot control pressure present on the second control output when the control lever is deflected by the same angle in the second direction. This can be attributed to the tolerances with which the individual components of a pressure reducing valve are afflicted. In particular, the tolerances of the control spring of a pressure valve influence the pilot control pressure.
The difference in the pilot control pressures at a specific pivoting angle of the control lever is not desirable in certain cases. In addition, there are applications in which, irrespective of the pivoting direction, the intention is for the same pressure to be present either on the first control output or on the second control output after a specific pivoting angle of the control lever. For example, a hydraulic control arrangement for a winch is known in which pivoting the control lever from its neutral position firstly adjusts a proportionally adjustable directional control valve from its mid-position in one direction or in the other direction, depending on the pivoting direction of the control lever. Beginning at a specific pivoting angle of the control lever, the feed diaphragm of the directional control valve is completely open. Starting at this pivoting angle, irrespective of the pivoting direction of the control lever, the absorption volume of a hydraulic motor driving the winch drum is then set as a function of the pilot control pressure. This adjustment is intended to take place starting at a specific pivoting angle of the control lever which can be sensed by the operator by means of a pressure point. Previously, a great deal of adjustment work has been needed if, firstly, it is wished to have the same pilot control pressure in the two control outputs in each case, irrespective of the pivoting direction, after a specific pivoting angle of the control lever, and if this pilot control pressure is also intended to have a specific value.
The invention is based on the object of developing a hydraulic pilot control system having the introductory-mentioned features in such a way that, irrespective of the pivoting direction of the handle, after a specific pivoting angle, one simply has a specific pilot control pressure at one of the control outputs. The aim is also to configure the known hydraulic pilot control system more cost-effectively.
The object is achieved wherein a hydraulic pilot control system having the introductory-mentioned features, has a pressure valve which, when the handle is pivoted in the first direction and when the handle is pivoted in the second direction, can be adjusted with the same effect, and has a directional control valve which, depending on the pivoting direction of the handle from a rest position, which it assumes in the neutral position of the handle, can be changed over into a first switching position, in which it connects the control output of the pressure valve to the first control output, or into a second switching position, in which it connects the control output of the pressure valve to the second control output. Therefore, in the case of a hydraulic pilot control system according to the invention, only one pressure valve is provided for two pivoting directions of the handle. Therefore, tolerances in the components of the pressure valve no longer influence the difference in the values of the pilot control pressures after the handle has been pivoted by a specific pivoting angle. If the adjustment of the pressure valve as a function of the pivoting angle is made equal for both pivoting directions of the handle, then in each case the pilot control pressure is also irrespective of the pivoting direction. If it is wished to have a specific pilot control pressure at a specific pivoting angle, then it is necessary for only a single pressure valve to be adjusted. Furthermore, a directional control valve can generally be produced with less effort than a pressure valve having many individual parts. A hydraulic pilot control system according to the invention can therefore also be produced more cost-effectively.
For example, according to other features, in the rest position of the directional control valve, the two control outputs are relieved of pressure via a tank connection of the directional control valve, circumventing the pressure valve. Intrinsically, in the rest position of the directional control valve, relieving the pressure on the control outputs would also be possible via the pressure valve, since in the neutral position of the handle, the control output of the pressure valve is relieved of pressure.
According to still other features, the directional control valve preferably has, as a movable control element, a rotary slide, whose axis is aligned with the axis of rotation of the handle and which can be rotated via the handle in a valve bore in a valve housing. Even at large pivoting angles of the handle, there are no difficulties here in connecting the control element of the directional control valve and the handle to each other. The rotary slide is advantageously urged by an axial stop against a stop on the valve housing by a spring, so that it always assumes the same axial position and reliably controls the connections between individual ducts opening into the valve bore.
A refinement according to the invention is particularly preferred wherein there is a permanently set pressure reducing valve, which serves for the internal control pressure supply and which is accommodated in a space-saving way in an axial bore in the control element of the directional control valve, formed as a slider. One speaks of an internal control pressure supply when a pressure reducing valve generates from a high system pressure a significantly lower control pressure, which is fed to an adjustable pressure valve.
According to another feature of the invention, the relationship between the displacement of the pressure valve, at least over a large angle range, when the handle is pivoted in the second direction from the neutral position is the same as when the handle is pivoted in the first direction from the neutral position. This may easily be achieved by a control disk for the pressure valve which is appropriately configured and can be rotated by the handle. At the same pivoting angle, therefore, irrespective of the pivoting direction, in each case the same pilot control pressure is present on one of the two control outputs. This is particularly advantageous if a hydraulic appliance is to be controlled in the same way irrespective of the pivoting direction of the handle.
A specific pilot control pressure at a specific pivoting angle may be set in a particularly simple way by the pressure valve being adjustable from the outside after its components have been mounted in a housing. In this case, two advantageous possible ways of adjusting the pressure valve are provided. According to one way, an adjusting spring is provided, whose prestress can be varied by means of an adjustable stop, so that a total spring force which results from the force of the control spring and from the force of the adjusting spring and acts on the control element may be adjusted. According to another way, for the purpose of adjustment, the control edges fixed to the housing are displaced axially, so that the control position of the movable control element and, therefore, at a given axial position of the plunger, the prestress of the control spring in the control position of the control element is changed. By means of a combination of the features of these two ways, both the level of the pilot control pressure at a specific pivoting angle of the handle, and also the idle angle between the neutral position of the handle and the start of a pilot control pressure build-up can be adjusted.
A refinement according to the invention is particularly preferred wherein the plunger is guided in a guide sleeve. The control cartridge, which is inserted adjustably into a housing in order to vary the position of the control edges fixed to the housing, is extended beyond the control edges and accommodates the guide sleeve in a captive manner. The control cartridge, the guide sleeve and the movable parts of the pressure valve therefore form a structural unit, which can be handled as a whole and can be simply mounted as a whole in a housing. Reference is expressly made to the fact that the configuration of a pressure valve according to this refinement is also advantageous when this pressure valve is used in conventional pilot controllers, in which there is generally an adjustable pressure valve for every pivoting direction of the handle.